Conventionally, a disk-shaped antenna is mounted in a device with GPS (Global Positioning System). The disk-shaped antenna is advantageous for being small and robust, having high heat resistance and low power loss, and being highly adapted to mount on a curved surface of a member. FIG. 1 shows a disk-shaped antenna 1 mounted in a cellular phone with GPS or GSM (Global System for Mobile communications) feature. The disk-shaped antenna 10 comprises a dielectric substrate 11 formed of ceramic material, a ring-shaped metal loop 12 formed on top of the substrate 11 by carrying out photolithography and etching, and a ground metal face 13 formed on bottom of the substrate 11 by carrying out photolithography and etching. Note that the shape of the metal loop 12 may take another form such as square or rectangle in other configurations. The metal loop 12 comprises two diametrical metal bands 121 and 122 across each other at its center. A hole (not shown) is provided in about center of each of the substrate 11 and the ground metal face 13. A coaxial 14 has a central conductor 141 inserted through the holes from bottom to top so as to expose its end 143 and which is in turn connected to a feeding point 123 at an intersection of the metal bands 121 and 122. Also, the braided outside conductor 142 is connected to the ground metal face 13. This finishes the manufacture of the disk-shaped antenna 10. As such, signal transmitted by the central conductor 141 of the coaxial 14 may be transmitted to the air or signal may be received by the central conductor 141.
As to the disk-shaped antenna, its resonant frequency is varied by the shape and size of the metal loop 12, input impedance is varied by the location of the feeding point, and spiral of electromagnetic wave is varied by an angle θ between the intersected metal bands 121 an 122. Thus, typically the shape of the metal loop 12 is decided in advance depending on practical needs in the manufacturing process of disk-shaped antenna. However, design parameters are difficult of being precisely controlled in the manufacturing process. As a result, properties of the produced disk-shaped antenna are typically not the same as desired or even completely cannot meet the requirement. Hence, an employee of a disk-shaped antenna manufacturer has to manually perform a fine adjustment on the metal loop 12 of the disk-shaped antenna 10 after manufacturing the disk-shaped antenna. This is a time consuming and labor intensive procedure. As a result, the manufacturing cost is increased greatly and thus mass production is made impossible.
In addition, the manufacturers have to produce metal loops 12 of different specifications for being adapted to cooperate with substrates 11 and ground metal faces 13 of different specifications for meeting the needs of disk-shaped antenna customers. As such, disk-shaped antennas 10 having different operating frequencies, impedances, and spirals of electromagnetic wave are produced, thereby fulfilling the needs of customers. This inevitably can adversely increase inventory and cost. Thus, a need for improvement exists.